A soybean oligosaccharide is a general term for oligosaccharides that are contained in a soybean such as sucrose, raffinose, and stachyose; these have an activity of activating useful bacteria in the intestines and therefore the use thereof mainly as a health food product has drawn much attention. Abundant soybean oligosaccharides are contained also in waste materials such as soybean whey and soybean molasses that are generated in large amounts in an industrial soybean processing step; the waste material such as soybean whey or soybean molasses is therefore expected to be utilized as inexpensive sugar raw materials as well.
As a method for producing monosaccharides or alcohol using the soybean oligosaccharide that is contained in waste materials such as soybean whey or soybean molasses, what has been disclosed are, for example, a method of allowing galactosidase to act on soybean oligosaccharides to obtain monosaccharides (see Non-patent Document 1) and a method comprising subjecting soybean molasses directly to fermentation to produce ethanol or butanol (see Non-patent Document 2).
Further, a method using soybean whey or a defatted soybean as a raw material has been known as a conventional production method for soybean oligosaccharide. Soybean whey is a waste liquid that remains behind after soybeans are subjected to steam cooking to yield broth, that is, soy milk and then proteins are precipitated and removed therefrom by addition of an acid or the like, and is known to contain soybean oligosaccharides, lipids, soluble proteins and the like. As a production method for soybean oligosaccharides using soybean whey as a raw material, what has been disclosed are, for example, a method comprising adding calcium hydroxide to soybean whey, heating the mixture, and precipitating and removing impurities to obtain oligosaccharides (see Patent Document 1) and a method comprising heating soybean whey, adding phosphoric acid thereto to lower the pH, and precipitating and removing impurities to obtain oligosaccharides (see Patent Document 2).
Further, defatted soybeans are a residue obtained by removing lipids from soybeans using a solvent such as hexane. As a method for producing soybean oligosaccharides using the defatted soybean as a raw material, what has been disclosed are for example a method comprising extracting a concentrate that contains Bifidobacterium-proliferating substances from defatted soybeans using an aqueous solution of alcohols to obtain oligosaccharides (see Patent Document 3) and a method comprising extracting oligosaccharides by adding water to defatted soybeans and obtaining soybean oligosaccharides from an extract liquid (see Patent Document 4).